Thermoelectric cooling (TEC) system is advantageous over conventional refrigerating system in many aspects, such as being more compact and involving only solid-state components. The basic concept behind TEC is the Peltier effect, which occurs when electric current flows through two dissimilar materials. Depending on the current flow direction, cooling or heating could occur at the junction between the two dissimilar materials.
The Peltier effect may be understood using an energy band diagram as illustrated in FIG. 1. Two dissimilar materials I and II have different energy levels. When the two materials are in contact, a conduction band discontinuity ΔEc may be formed at the junction. When an electric field is applied to the material system, the conduction band energy level for electrons is slanted, as shown in FIG. 1, in a direction opposite to the electric field direction.
Electrons in a material have a distribution of kinetic energy following certain statistics. The temperature of the material may be characterized by an average kinetic energy of electrons in the material. An electron (e−), driven by the electric field, tends to move along the slanted direction of the conduction band. To cross the junction, the electron would have to overcome the potential energy barrier ΔEc. Consequently, among all the electrons, those of higher kinetic energy, i.e., contributing more to the temperature, have higher probability of crossing the junction.
Because higher-energy electrons selectively cross the junction and leave material I, the average energy of the electrons left in material I near the junction is effectively lowered, resulting in a cooling near the junction between materials I and II. When the electrons cross the second junction between materials II and III, the average kinetic energy of electrons in material III near the second junction becomes higher, so does the temperature near the junction. Thus, applying an electric field across the material system shown in FIG. 1 effectively transfers heat in the direction of the electron motion, from the first junction to the second junction.
Among the materials used for TEC technologies, Bismuth Telluride is the most widely used because it can be more easily optimized for pumping heat. In addition, because Bismuth Telluride is a semiconductor, it can be easily doped to be either n-type (electron conducting) or p-type (hole conducting). The simplest TEC module can be constructed using a single semiconductor “pellet,” which is soldered to electrically-conductive materials on each end. The electrically-conductive material used is usually plated copper. In this configuration, materials I and III are, in fact, the copper connection paths to the power supply. Such a simple TEC module is illustrated in FIG. 2A. When operating the TEC module of FIG. 2A, electrons flow in the wire 21 in the direction shown by the arrows, driven by the electric field from the DC voltage source 22. When electrons cross the junction between the copper plate 23 and the n-type semiconductor pellet, heat is absorbed at the junction. When the electrons cross the junction of the opposite side, heat is released. Note that the heat transfer direction in the n-type module is along the electron motion direction, which is opposite to the electrical current direction.
Similar to the n-type TEC module shown in FIG. 2A, FIG. 2B illustrates a p-type TEC module. The same or a different power supply 22 may be used to drive an electrical current originating from the “+” terminal of the voltage source, through the wire 25 to the copper plate 23. In the p-type pellet 26, the electrical current is carried by holes. Note that in the p-type TEC module heat transfer is in the direction of the motion of holes, which is the same as the electrical current direction. Such a property, resulting from the fact that electrons and holes have opposite charges, is advantageously used in building a practical TEC device as shown in FIG. 3A, wherein a plurality of n-type modules 31, 33, and 35, and p-type modules 32, 34, and 36 are electrically connected in series, while heat is pumped in a direction parallel to the arrangement of the modules. The heat transfer direction is the direction of the electron flow direction, which is the same as the hole flow direction.
As illustrated in FIG. 3B, a conventional TEC device consists of 254 alternating p-type pellets 32 and n-type pellets 31 arranged in a 2-D array. The pellets (modules) are paired and electrically connected using copper plates 37. The array is usually disposed between ceramic substrates 38 and 38′. The heat transfer direction is indicated by array 39. Such a device can be driven with a 12-16 V DC power supply and draws a current of 4-5 amps.
The compact design of the multiple pellets results in a high efficiency for heating from the cold side to the hot side. Subsequently, the temperature of the hot side increases and heat dissipation in the hot side must be taken into account when designing a practical TEC system.